This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-200123, filed Jun. 30, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of correcting a projection optical system and a method of manufacturing a semiconductor device by using such a correction method.
2. Description of the Related Art
At present, the reduction projection exposure system used in the manufacture of semiconductor devices entails the problems of deformation of device patterns and deterioration of the focus latitude due to residual aberrations in the projection lens. As a solution to these problems, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-52582 proposes a technique for correcting such adverse effects to device patterns due to the residual aberrations.
The invention discussed in the above publication relates to transfer of a device pattern with use of an alternating phase shift mask, and correction measures in this invention are carried out as follows. That is, a mask pattern used for detecting a comatic aberration is projected to a resist at a predetermined reduction rate. Then, the projected resist pattern and a pattern obtained by reducing the original mask pattern at the predetermined reduction rate are compared with each other in order to measure the degree of the effects of the comatic aberration. The mask pattern is corrected on the basis of the measured result.
The technique of the invention disclosed in the above document is not at all different from a case where the condition setting operation generally used for obtaining an optimal mask pattern is applied to an alternating mask in the case where an actual device pattern is used as a mask pattern for detecting the comatic aberration. In the condition setting operation for the optimal mask pattern in an ordinary binary mask, correction for the effects of aberrations is carried out at the same time as correction with regard to an optical proximity effect (OPE) or a resist process effect. This is because these operational steps cannot be separated from each other. It is a well-known fact that an alternating phase shift mask entails lager aberration effects than that of a binary mask. From this fact, it is only natural that the ratio of the operation of correcting the effect of aberrations becomes relatively high in the condition setting operation for an optimal mask pattern in an alternating phase shift mask. For this reason, it is considered that the invention disclosed in the document emphases such a fact.
Meanwhile, the degree of the effects of aberrations, expressed by, for example, the deformation amount of a pattern, varies widely depending on the type of mask (such as binary mask, attenuated phase shift mask, and alternating phase shift mask), illumination conditions (large "sgr" illumination, reduced "sgr" illumination, annular illumination and quadruple illumination), design of pattern (shape, design rule and critical dimensions). Therefore, it is not always true that the larger the aberration itself, the greater the effect of the aberration, or the smaller the aberration itself, the smaller the effect of the aberration. Thus, nothing can be gained, in practical terms, as long as not the effects of the aberrations are examined, in stead of the aberration. Further, it is theoretically absolutely impossible to have a projection lens completely free of aberrations. Thus, as long as optical lithography is employed, the problem of aberrations will inevitably remain.
Under these circumstances, there can be proposed several methods as techniques for correcting the effects of aberrations in the actual manufacture of semiconductor devices.
First, as in the case of the published document presented above, there is a method for correcting a design of a mask pattern in consideration of the effects of aberrations. In this method, at first, a device mask is formed in consideration of an ideal situation, and on the basis of the result obtained by transferring the pattern of the thus obtained device mask onto a substrate, a final device mask is formed. Therefore, this method entails such a problem that the accuracy of the firstly formed mask has a significant influence on the correction accuracy of the final mask. Further, when the illumination conditions are changed in the middle of an operation, the data handled up to the time of the condition change becomes unusable, and therefore the operation must be started all over again from the beginning. Furthermore, the effects caused by aberrations may vary due to slight differences in dimensions. Therefore, in order to make a mask design to fall within an allowable range, it is considered that the correction of mask must be repeated a number of times.
Second, there is a method of varying the illumination conditions such as to lessen the effects of aberrations. However, the initial illumination conditions are set such as to maximize the exposure latitude, and therefore the latitude is reduced after changing the illumination conditions as compared to the transfer of a pattern under the initial illumination conditions. Further, it is not clear whether a practical solution is present or not, and therefore the method itself cannot be certain at all.
Further, each of the above-described methods is time consuming. Regarding the semiconductor devices manufactured at present as well as in the future, there is a general tendency for producing small batches of a variety of types, and the number of types of device patterns will be drastically increased. Therefore, it is expected that there will be a variety of types of effects on device patterns due to aberrations, and it will become necessary to realize an aberration correction method capable of quickly responding to the effects of aberrations.
As described above, with regard to the projecting exposure devices, the residual aberrations of a projection optical system are becoming an increasingly serious problem. However, with the methods of correcting the mask pattern or illumination conditions, a quick alteration in the processing is difficult to accommodate.
According to the first aspect of the present invention, there is provided a method of correcting a residual aberration of a projection optical system, which is used for projecting a pattern of a photo mask onto a photosensitive film located on a substrate, the method comprising: calculating an effect of a residual aberration on a given pattern on the basis of the residual aberration of the projection optical system obtained by measurement, calculating a moving amount of an adjustable aberration in the projection optical system such that the effect of the residual aberration becomes minimum in a given area, and moving the adjustable aberration in accordance with the calculated moving amount.
According to the second aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising correcting a residual aberration of a projection optical system, using the correction method described above, and projecting a pattern of a photo mask onto a photosensitive film located on a semiconductor substrate by the projection optical system.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.